1. Field of the Invention
The present invention relates generally to measuring systems for radio-frequency communication systems and specifically to measuring devices for the measurement of passive intermodulation. In particular, the present invention relates to a method for intermodulation measurement such as a method for locating points in a signal transmission path for a radio-frequency signal which are faulty with regard to radio-frequency transmission properties of the signal transmission path. The invention further relates to a method and a measuring device for bandwidth-optimized distance measurement in PIM measuring devices.
2. Description of Related Art
Description of Passive Intermodulation (PIM)
The quality of a connection between a fixed transmitting and receiving device (BTS, base transceiver station) and a terminal device (UE, user equipment) plays an important role in modern mobile networks. Due to the high generated transmitting power in the BTS on the one hand and the necessary sensitivity of the receivers of the BTS and UE on the other hand, faults in the transmission path can significantly influence the sensitivity of the receivers and thus the quality of the connection.
One key effect which causes interference in the transmission path is intermodulation. For example, through intermodulation, two transmission signals with two different carrier frequencies which are generated with high power in a BTS generate interfering signals at points with non-linear transmission behavior (often simply referred to as “non-linearities”), the frequencies of which are sums and differences of integral multiples of the frequencies of the transmission signals. Some of these interfering signals can fall within the reception band of the BTS and thus adversely affect the quality of the communication. If these interfering signal are generated in passive elements, this is referred to as passive intermodulation (PIM).
FIG. 1 is a schematic representation showing a signal transmission path from a BTS up to an antenna. The BTS 10 is connected with the antenna 13 via a first filter 11 and a second filter 12. The BTS 10, the filters 11 and 12 and the antenna 13 are connected together via radio-frequency cables 14, 15 and 16, which are connected to the respective elements via radio-frequency connectors 17 to 22. PIM can occur in all components 11 to 22 of the transmission path. For example, corrosion in plug connectors, oxide coatings on contacts and metal-metal transitions, impurities in materials and insufficiently fastened plug connections can cause PIM.
In order to ensure and check the quality of the transmission device, measurements of the PIM are carried out. Since PIM occurs in particular at high powers, as a rule this is measured with the use of high transmitting power, for example 2*20 W. It is of particular importance for measurement of a transmission installation as shown in FIG. 1 to be able to localize a possible fault along the transmission path from the BTS 10 up to the antenna 13 in order to identify defective components in the transmission path and specifically eliminate the fault. A high resolution, for example into the 10 cm range, greatly simplifies fault location.
The publication DE 199 46 763 A1 describes a method for measuring cable properties of a telecommunications cable. An echo signal reflected from the end of the cable is thereby evaluated. This method cannot be used to locate points with non-linear transmission behavior.
Points with non-linear transmission behavior can be located through the method described in the publication DE 10 2012 023 448 A1. However, this method involves considerable complexity in terms of measuring, since it requires that several RF signals be generated and an intermodulation product derived from these, and that a cross-correlation between intermodulation product signals be carried out. Due to its complexity, this method can lead to an insufficient resolution in terms of location.
Structure of PIM Measuring Devices
The structure of a known PIM measuring device is described in the publication DE 10 2010 015 102 A1. Such a measuring device is represented in FIG. 2 and will be explained briefly in the following.
PIM Distance Measurement
Signals with the frequencies f1 and f2 are generated in two signal sources 101 and 102, whereby the signal usweep from the signal source 101 is periodically swept over a defined frequency range Δf starting with the frequency f1. The frequency of the signal usweep is illustrated by way of example in FIG. 3. The signal usweep is modified in the mixer 111 through multiplication with the signal from the signal source 102, doubled in the frequency doubler, in such a way that the resulting signal uref has the same frequency and signal form as the PIM signal generated in the signal transmission path or in the DUT 120. As a result of the runtime of the PIM in the signal transmission path over the path distance l, urx is received with a delay of dt in comparison with uref, where:dt=(2lεreff)/c0  (1)
c0 representing the speed of light, εreff the effective dielectric constant of the cable with the path distance l. FIG. 3 illustrates how the time delay dt in comparison with signal uref gives rise to a frequency difference df between uref and urx. The signal ub is generated through multiplication of the signal uref and urx in the mixer 112. The frequency of ub is a measure for the distance l of the interference signal from the measuring device. The calculation of the distance l from ub takes place in the evaluation unit 113.
Which Problem is Solved
The transmission devices according to FIG. 1 are bandwidth-limited systems. The filters 11 and 12 as well as filters within the BTS usually limit the bandwidth to the transmission and reception band relevant for the transmission device. Through the limited bandwidth on the one hand and the frequency modulation method, with its poor bandwidth efficiency, on the other hand, limits arise with respect to the resolution of fault location which limit the accuracy of the measuring method according to DE 10 2010 015 102 A1.
In view of the problems described, the invention is based on the object of modifying a method of the aforementioned type in such a way that points in a signal transmission path which are defective in terms of RF transmission characteristics can be located as precisely as possible, so that the troubleshooting is simplified, whereby the method should be simple and possible to carry out without a complex measuring set-up.